Plain English Meaning
A line on a chart that links all the places where the compass points the same amount off from true north.
Full Definition
A line drawn on an aeronautical chart connecting points that have the same magnetic variation — that is, the same angular difference between true north and magnetic north.
Context Anchor
Seen on aeronautical charts and in discussions of compass use, navigation, and magnetic variation.
Derivation
From Greek 'iso-' meaning 'equal' and 'gonia' meaning 'angle.' So an isogonic line is literally a 'line of equal angle' — every point on it shares the same magnetic variation angle.
Why Pilots Care
Gives the exact variation value needed to convert a or into the actually flown on the compass.